He did not know exactly how many people were affected - but it is thought the number could run into hundreds of thousands.

Those affected, including customers at 30 Co-op outlets across Northern Ireland, will have their money credited directly back to their accounts within the next 24 hours, the company has promised.

A spokesman for the Co-op said the mistake was down to a "processing error".

The Co-op has 2,800 food stores and 200 filling stations across the UK. It said all of them could potentially have been affected and any type of debit or credit card could have been involved.

Not everyone shopping on Tuesday with the Co-op will have been charged twice for the same transaction, and if people are unsure about whether they have been charged twice they should check their statement or their online account.

The Co-op's customer careline number is 0800 068 6727.

Ulster University retailing expert Don McFetridge said last night that this was a very serious and unusual error on the part of a major UK retailer.

He expressed concern that the double-charging blunder could put some customers into the red on their bank accounts - incurring extra charges from their banks.

"Not everyone uses online banking, so some Co-op customers may not find out about this double charging for weeks, and get quite a shock when they find they have incurred additional charges from their bank or credit card company," he said.

However, the Co-op has promised to reimburse people who have incurred charges as a result of the error, such as overdraft fees.

A Consumer Council of Northern Ireland spokeswoman urged Co-op customers to check their bank accounts and statements, and to keep any receipts or other documents relating to any double charging or additional finance charges incurred because of the Co-op blunder.

Affected customers seeking further advice may also contact the Consumer Council helpline on 0800 1216022 or visit its website at consumercouncil.org.uk